etching

Etching is the deliberate, controlled corrosion of a polished surface to make its structure visible. A perfect mirror polish reflects light uniformly and looks blank; etching lets a chemical eat at grain boundaries and different phases at different rates, carving tiny valleys and steps so that light scatters unevenly and the structure jumps into view — like breathing on a clean window to reveal fingerprints.

You swab or dip the polished sample in a specific reagent for a few seconds. Grain boundaries are higher-energy, more disordered regions, so they dissolve faster and become grooves that appear as dark lines. Different phases have different chemistries and dissolve at different rates or tint differently, so they show up as different shades. A classic reagent is nital (a few percent nitric acid in ethanol) for steels; each alloy family has its own recipe and its own timing.

Without etching you cannot see grains or most phases at all — the polish alone shows only cracks, pores and inclusions. But etching is destructive and time-sensitive: under-etch and boundaries are faint; over-etch and everything turns black and detail is lost. The etchant also biases what you see, since it reveals whatever it attacks — choosing the reagent is choosing which structure to make visible.